1. Field of the Invention
This invention relates to holographic scanner spinners.
2. Description Relative to the Prior Art
Holographic scanner spinners are devices which when positioned to intercept a stationary beam of light, such as from a laser diode, and rotated, cause the beam to scan. Holographic scanner spinners are known comprising a carrier substrate in the form of a disc, mountable for rotation about an axis at the center of the disc. The disc may be transparent or opaque depending on whether the holographic scanner spinner is of the transmission or reflection type. The disc may be regarded as divided onto a plurality of sector-shaped facets, that is, regions bounded by two radial lines extending radially from the axis of the disc and an arc concentric with the disc, the arc usually being a portion of the circular periphery of the disc. The angles included by the radial lines bounding each of the facets, are the same and sum to 360.degree.. Each facet includes a diffraction grating pattern usually formed in a photoresist layer carried by the carrier substrate disc. The lines of the diffraction grating pattern of each facet may be "radial", that is, parallel to the radius which bisects the facet, or they may be "tangential", that is, perpendicular to the radius bisecting the facet. Usually, in a multi-facet holographic scanner spinner, the diffraction grating pattern of a facet does not extend to the center of the disc. This is so because the cross-section of the beam, which the holographic scanner spinner is intended to intercept, has finite dimensions and hence its path on the holographic scanner spinner has to be spaced from the center of the disc otherwise the beam would always be incident on a plurality of facets, which, for many purposes, would be a useless condition. The diffraction grating pattern usually extends to the periphery of the disc or to a circle concentric with the disc and having a radius only slightly smaller than that of the disc. Thus, each diffraction grating pattern has usually had a shape bounded by parts of two radii and by inner and outer concentric arcs. Recently, there has been a proposal that each grating pattern should have an arcuate extent slightly greater than the arcuate extent of the facet so that the grating patterns of adjacent facets slightly overlap one another while the facets are only contiguous.
Many ways of making diffraction ratings are known. Ten ways are shown on page 101 of Diffraction Gratings by M. C. Hutley published in 1982 by Academic Press. In all of these ways, it is inherent that the larger the diffraction grating pattern, the larger must be the cross-section of the beam of light which is split and interfered with itself. One of the ways for producing diffraction gratings disclosed in the aforementioned book, is illustrated in FIG. 1 of the accompanying drawings. It will be observed that a beam 20 of coherent radiation from a laser, is directed at a prism 22. The portion of the beam 20 incident on facet 24 of the prism 22 is refracted downwards and the portion of the beam incident on the facet 26 is refracted upwards. The two portions of the beam interfere in the air space, contiguous with the base 28 of the prism, after they emerge from the base 28, and create a straight line interference pattern in that air space. Designated by 30 is a plane, parallel with the base 28 of the prism 22, in which lies the interference pattern having the greatest area. It will be observed that the maximum dimension d of the diffraction pattern which can be created on the plane 30 is directly related to the dimension D of the laser beam.
Obviously if the interference patterns can be made smaller, the optical system used to create the interference patterns can also be smaller and hence less costly and probably more accurate.
FIGS. 2 and 2a illustrate a known holographic scanner spinner 32 comprising a substrate carrier disc 31 of rigid material having thereon a coating 29 of photoresist material. The disc 31 has a central aperture 27, for cooperation with a mounting and drive shaft (not shown) on an axis 33. The holographic scanner spinner has six facets 34, bounded by radial lines 35, each having an included angle of 60.degree.. Each facet 34 includes a diffraction grating pattern 36 bounded by arcs of inner and outer circles 38 and 40, respectively, which are concentric with the holographic scanner spinner 32. The diffraction lines 42 of one pattern are represented in FIG. 2, on a greatly enlarged scale, of course, and are shown in one facet only. It will be observed that the lines 42 are parallel to the radius 43 which bisects the facet of which they are part. Thus, the illustrated holographic scanner spinner is a "radial holographic scanner spinner".
FIG. 3 illustrates a known mask 44 for defining the extent of the diffraction grating pattern to be created in each facet 34 of the holographic scanner spinner 32. The mask has an aperture 46 which is identical in shape and size to the diffraction rating pattern to be created in each facet. The mask 44 also has a center point 48 which is the point of intersection of the lines of the two sides 47 of the aperture.
For making a holographic scanner spinner, the mask 44 is placed against a substrate carrier disk coated with photoresist and with the center 48 of the mask 44 coincident with the axis 33 of the disc. The prism 22 is so disposed that the disc takes the place of the plane 30 illustrated in Fig. 1, and the interference pattern fills the aperture 46. The edge 23 of the prism is disposed so that it is parallel to the radius 43 bisecting the facet. The prism and the dimension D of the light beam 20 are both big enough that the entire aperture 46 is illuminated and an interference pattern is created on the entire area of the photoresist uncovered by the aperture in the mask 44. After exposure, the disc is rotated through exactly 60.degree. and another exposure is made. This step and expose process is repeated until all six facets have been exposed. Thereafter the photoresist is etched and a holographic scanner spinner results.
FIG. 4 represents one facet 34 of the holographic scanner spinner which, for the purpose of the description, will be regarded as rotating counterclockwise, as indicated by the arrow 50. Also shown in FIG. 4 is the spot 52 created by the collimated beam of laser light which is to be converted from a stationary beam into a scanning beam by the rotating holographic scanner spinner. In this example, the spot is elliptical but, as is known, the spot may have other shapes.